Hawaii

The Island of Hawai‘i lies over or just north of the Hawaiian
hot spot and is composed of five volcanoes and one active seamount:
Kohala, Hualälai, Mauna Loa, Kïlauea, Mauna Kea and Loihi
located offshore. Of these, only Mauna Loa, Kïlauea, and Loihi
are considered active, while Haulälai is dormant with its
most recent eruption ending sometime in 1800-1801. The island has
428 km of general coastline and is so large relative to the other
Hawaiian islands, it is known locally and abroad as the Big Island.

On the Big Island, well-developed black and green sand beaches
signify the active reworking, by waves and currents, of the freshest
lavas in the state. The island’s youth has, in general, allowed
for a lower degree of beach formation along its rough volcanic
coastline. White calcareous beaches make up a relatively small
component of the shoreline largely because of poor reef development
due to recent active coastal volcanism.

The orographic effect of the Big Island’s large shields:
Mauna Loa (4,169 m), Mauna Kea (4,205 m), and Kohala (1,670 m)
create a lush region on the northeast side of the Big Island, where
annual rainfall is 150–400 cm.

The
Puna District comprises the eastern most corner of the Big Island, reaching
out at Cape Kumakahi, a broad rocky point with a shallow slope at the
coast. This area was resurfaced during the 1960 flow originating from
Kïlauea’s east rift zone. The flow narrowly missed the navigational
light on the point as it remarkably divided into streams and flowed around
the lighthouse (Clark, 2002).

North
of Cape Kumukahi the coast extends for roughly 25 km around Leleiwi Pt.
into the Hilo District and Hilo Bay in a series of wave weathered low
rocky sea cliffs, and rough remnant lava flows . Hilo Bay is one of two
deep draft harbors on the Big Island. The bay is the seaward end of the
Wailuku River valley that runs along the junction between the younger
Mauna Loa lavas to the south and older Mauna Kea lavas to the north.
A breakwater 3 km long extends offshore from a large natural deep water
gap in the fringing reef of Kühiö Bay. The structure runs west
in front of the developments fronting eastern and central Hilo Bay. Hilo
is a heavily populated coastal city that was devastated in both 1949
and 1960 by tsunamis originating as earthquakes in the Aleutian trench
and along the Chilean coast respectively (Fletcher et al. 2002). In 1984
Mauna Loa broke a nine-year period of quiescence sending lava flowing
down to the northern city limits, a reminder of the volcanic hazard in
this coastal city (Fletcher et al. 2002).

The Pepe‘ekeo-Hämäkua Coast stretches for 100 km between
Hilo and Waipi‘o Valley to the north. This northeast and northern
exposed coastline consists of a steep rocky shoreline of Mauna Kea lavas,
characterized by a multitude of headlands, sea cliffs (~30-90 m high),
coves, and irregular embayments such as: Onomea, Hakalau, and Maulua
bays. The Hämäkua coastal embayments are seaward ends of stream
carved gulches that originate upland on the lush Mauna Kea mountainside,
and are a passage to the coast for water rich with sediment, soil, and
eroded volcanic rock. Black volcanic pebble and cobble stone beaches
line the bay heads, and large blocks of fluvially transported basaltic
debris along the shoreline lie where they have been scattered by high-energy
waves. No fringing reef has developed along this relatively young shoreline,
however offshore rocky islets are commonplace along the northeast coasts
of the Big Island; totally exposed to north Pacific trade wind seas.

Waipi‘o
Valley, in the far northeast, marks the junction where Mauna Kea lavas
to the south overlie lavas of the broad and elliptical Kohala Volcano
(1,670 m) to the north. Kohala is the Big Island’s oldest shield,
projecting northwestward and forming the northernmost portion of the
island. Waipi‘o Valley is the first, and most immense (approximately
1 km wide), of seven spectacular shore-normal amphitheater valleys that
extend northwest along the Kohala coast. These valleys developed during
a lower stand of the sea (~90 m), and were partly filled with alluvium
at that time. As sea level rose the alluvium was reworked and the modern
low lying, scenic wetland valley floors that lie inland of the coast
were created (Macdonald et al. 1986).

The
valleys, such as: Waimanu, Honopu‘e, Honokäne Nui/Iki and
Pololü, are bordered by steeply ascending massive rock walls. Except
for a four-wheel drive road into Waipi‘o, access to the lush valley
floors is limited to foot trails or boats. Dynamic beaches dominated
by black volcanic sand, line the seaward end of the northeast Kohala
valleys. Between the largest of the amphitheater valleys, the coast is
dominated by blunt sea cliffs that rise to 400 m above the ocean, the
truncated remains of the shield that once extended at least a kilometer
into the ocean. The cliffs are incised by numerous stream cut canyons
300-750 m deep that form a series of narrow coastal hanging valleys (Macdonald
et al. 1986). This area represents the only coastal segment of Kohala
that was spared resurfacing by the Hawi Volcanic Series toward the end
of the Kohala main shield building stage when Hawi lavas flowed over
most of the eroding shield (Macdonald et al. 1986).

North
of Pololü Valley, the east Kohala coastline is made up of steep
lava headlands and irregular low lying rocky embayments where streams
enter the ocean out of wetland gulches. Like the shoreline to the south,
small rocky sea stacks lie offshore of eastern Kohala, isolated from
the retreating coast by heavy North Pacific surf (Fletcher et al. 2002).
This coastal morphology, with the addition of sparsely distributed cobble
and boulder beaches, extends from the exposed northern tip of the island
at ‘Upolu Pt. around to Kawaihae Bay and Harbor at the western
intersection of the Kohala and Mauna Kea shields.

The
western side of the Big Island lies in the lee of Mauna Loa (4,205 m),
the largest volcano on Earth (measuring from the sea floor), which has
formed in the last 600,000-1,000,000 yrs, rising almost 9 km from the
sea floor. The leeward climate is extraordinarily dry with 25 cm annual
rainfall, and minimal stream erosion on the Hawi lavas of western Kohala,
and on the lavas of Mauna Kea, Mauna Loa, and Hualälai, south along
the coast respectively. Kawaihae Harbor is the second deep draft harbor
on the Big Island and is fronted by a system of offshore fringing reefs.
Several sand beaches exist at the south end of Kawaihae Bay, derived
from eroded coral that was dredged during construction of the harbor.

Approximately 5 km south of Kawaihae, along a shoreline of bright golf
course greens and hotel development, lies Häpuna Beach, the widest
of the few well-developed calcareous sand beaches on the Big Island.
The Puakö coast, lies just south of Häpuna Beach, along the
coast that was extended by historic Mauna Loa eruptions. Renowned for
its tide pools at the shoreline and well-developed offshore fringing
reef, the Puakö coast is a popular scuba dive destination.

Hualälai Volcano (2,521 m) makes up the central western coast
of the Big Island. Now a dormant volcano, it last erupted in 1800–1801,
burying an ancient Hawaiian village along the northwest coast. From the
western banks of the shallow coastal lagoon at Kïholo Bay, the Hualälai
coast extends southwest and is rocky and shallow sloped. Offshore fringing
reef and pocket sandy beaches line the numerous embayments including:
Kakapa, Kua, and Mahai‘ula Bays along this coast.

Lavas
of the most recent Hualälai eruptions make up the coast from the
south side of Mahai‘ula Bay to the western tip of the island at
Keähole Pt. This shoreline is characterized by low rocky headlands
fronted by fringing reef, small rocky remnants offshore, and beautiful
tide pools and beaches (both black and white) along the shore. The coastline
of historic Hualälai lava extends less than 30 km south of Keähole
Pt., beyond Kailua-Kona to the small non-distinct west facing Kuamo‘o
Pt. Lavas of the southwest Mauna Loa rift zone extend beyond Kuamo‘o
Pt. to the south point of the island. The South Kona district has a coastline
rich with relics of historic Hawaiian habitation. Much of this coast
is low-lying historic lava that has been gradually invaded by hardy vegetation.
In contrast, Kealakekua Bay is an area along this coast that contains
steep cliff walls that reveal the layered nature of their basalt. The
bay is accessable only by boat or foot trail and harbors one of the State’s
underwater parks. This 315-acre marine conservation district is lined
with a vibrant healthy reef ecosystem (Clark 2002). Abundant fish swim
about the shallow corals that dip steeply away from shore toward deeper
water. The bay is the location where Captain Cook moored for reprovisioning,
and ultimately his death.

Between
Ka‘ohe and Moili‘i, streaks of relatively recent flows (1950’s)
extend to the coast from the volcano. Here tide pools at the shoreline
are generally formed of lava spits in various stages of erosion. These
areas are mainly accesable by 4x4 roads and are devoid of development.

Because of its relative youth, the Mauna Loa coastal terrace of the
southwest coast has lower degrees of soil development. In its absence,
subsurface streams fed by upland precipitation penetrate the relatively
young porous basalt. These conditions have created a system of sub-surface
fresh water flows that feed wetlands and offshore freshwater seeps along
southwest coast of the Big Island from Miloli‘i to Ka lae Pt. The
shoreline from Moili‘i to Ka lae Pt. is made up of shallow and
moderately sloped headlands and low sea cliffs, and is backed by numerous
wetlands areas fed by the underground freshwater flows. Beaches of the
southwest coast are almost exclusively black sand. Several cones of cinder
and ash lie just inland of the southwest shore and were built by littoral
explosions during historic sea entries of Mauna Loa lavas (Macdonald
et al. 1986). The most recent flows of 1868 produced Pu‘u hou a
cinder cone rising 72 m from the shoreline. Wave erosion has truncated
the cone, resulting in a beach of red cinder at its base. Olivine sand
beaches skirt the rocky headland coasts of south Hawai‘i produced
by the fresh erosion of the island’s basalt.

Extending
north from Ke lae Pt. the southeast coast is made up of steep rocky headlands
that transition to a low-lying coastal plain in the region of Honu‘apo
Bay. This geologically complicated area has been impacted by historic
tsunamis and general tectonic subsidence. The Kïlauea southwest
rift zone cuts through the coastline into the seafloor several kilometers
northeast of Waio‘ala Spring, at Pälima Pt. marking the end
of the wetland areas of south Mauna Loa and signaling the start of the
Kïlauea coastline.

Kïlauea Volcano is a large bulge on the southeast flanks of Mauna
Loa although it is an independent volcano with its own magma plumbing
system. This volcano has been consistently active since at least the
early nineteenth century, if not since its emergence above sea level,
continuously creating and redefining the coastal areas along east and
southeast sides of the Big Island. On

May
12 th 2002, the Mother’s Day Flow, commenced on the south flank
of Kïlauea after issuing from a new vent near the southwest base
of Pu‘u ‘ö‘ö cone, 8 km upslope from the
south east coast. As is typical of Kïlauea flows, the Mother’s
Day Flow travels as molten lava through preexisting lava tubes down the
south flank of the volcano to the coastal plains. By late July 2002,
two arms of the flow had reached the ocean, forming a broad lava delta
that runs 570 m along the coast extending 50 m offshore of the West Highcastle
coast, and a 1,540 feet wide lava bench that extends more then 300 ft
offshore at Wilipea. Lava benches, such as at Wilipea, are a mechanism
of island growth. These form as lava builds up repeatedly into thick
and wide benches at the coast. Lava benches may collapse suddenly into
the ocean, leaving behind sharp cliffs, and creating steam plumes of
lava haze as seawater boils and vaporizes on contact with the rocks that
may be hotter then 1100º C. These collapses may release water vapor,
sulfur dioxide, chlorine gas, and fine shards of volcanic glass. Activity
at the surface of Kïlauea reflects the dynamics at depth within
the magma chamber that are recognized as earthquakes on the surface.
They may mark movement of the south flank, as it shifts steadily seaward
at a rate of about 8-10 cm per year (Morgen et al. 1998).